Inactivation of the p53 tumor suppressor is an important step in malignant transformation; over 50 percent of human tumors lose p53 function by mutations or deletion. Wild type p53 in the remaining tumors may also be impaired by other mechanisms such as overexpression of the MDM2 oncoprotein, or loss of the tumor suppressor ARF. Wild type p53 in a subset of tumors (greater than 95 percent of undifferentiated neuroblastomas and 37 percent of inflammatory breast tumors) is localized exclusively to the cytoplasm, and this may represent a unique mechanism of p53 inactivation. In studying tumor cell lines with cytoplasmic wild type p53, we have found that nuclear p53 accumulation can be induced by antisense inhibition of MDM2 expression, inhibition of the nuclear export pathway by leptomycin B, expression of the MDM2 inhibitor ARF, or spontaneous differentiation. These results suggest that MDM2 is responsible for inhibiting nuclear p53 accumulation in these tumors, in the absence of gene amplification or overexpression. We hypothesize that in neuroblastomas, MDM2 regulates p53 subcellular localization by promoting its nuclear export, which is in turn regulated by ARF and cell differentiation. We propose to test this hypothesis in the following experiments: [1]. Determine whether loss of ARF expression causes p53 nuclear exclusion in neuroblastoma cell lines. [2]. Determine whether nuclear translocation of p53 after differentiation is caused by changes of MDM2 or ARF expression and function. [3]. Determine whether the phosphorylation status of p53 and MDM2 is altered after differentiation, which inhibits MDM2-mediated nuclear export and degradation. [4]. Determine whether nuclear accumulation of p53 after differentiation increases p53 activity and sensitivity to DNA damage. These experiments will lead to a better understanding of the mechanism of p53 inactivation by nuclear exclusion, the regulation of MDM2 by ARF, and the regulation of p53 by differentiation.